During the last year, we continued define the molecular events that regulate hematopoietic stem cell (HSC) quiescence, survival, self-renewal and, myeloid cell lineage commitment and differentiation. We have focused our efforts on transcription factors since they are essential for stem cell and lineage development and, are often deregulated during the development of leukemias and lymphomas. Hematopoietic transcription factors regulate the expression of hematopoietic growth factors (HGF), HGF receptors, other transcription factors, and lineage specific genes. In this regard, we have discovered that the helix loop helix transcription factor, Id1, is not expressed in normal HSC and common lymphoid progenitors (CLP), but is expressed in more committed common myeloid progenitor cells (CMP) There are four Id proteins (Id1-Id4), which function as dominant negative regulators of other helix loop helix transcription factors as the Id proteins lack the basic region required for DNA binding. The expression of Id1 increases as CMP undergo further maturation and commitment into granulocyte macrophage progenitors (GMP). Finally, we found that neutrophils express low levels of Id1, while terminally differentiated macrophages express high levels suggesting that Id1 may regulate cell fate in GMP. We observed that HGF including interleukin 3 (IL-3), which promote myeloid cell differentiation induce Id expression in HSC and CMP, while HGF that promote erythroid and B cell development do not regulate Id1 expression. Thus, HGF instruct hematopoietic stem cells toward a myeloid versus erythroid and lymphoid cell fate through the induction of Id1. We confirmed these observations in vivo by transplanting HSC, which over express Id1, into irradiated recipients. We found that over expression of Id1 in HSC inhibited B cell development, impaired erythroid development, and enhanced myeloid development. Specifically, we observed an increase in macrophages in the bone marrow and peripheral blood, and increased numbers of granulocytes in the peripheral blood. In addition, we found that all of the mice transplanted with Id1-infected bone marrow cells became moribund beginning at six to eight months. Histological analysis of moribund mice indicated that these mice had increased extramedullary hematopoiesis and hyper proliferation of myeloid cells, suggesting that these mice developed a myeloproliferative disease (MPD). We are currently investigating if this disease is transplantable, and if these mice develop leukemia. We also found that infection of BMC with Id1expressing retrovirus increased the frequency of myeloid colonies in soft agar, suggesting that Id1 can enhance myeloid cell proliferation in vitro. These colonies contained increased numbers of immature myeloid cells in addition to differentiated granulocytes and macrophages. Finally, immortalized cell lines could be derived by culturing Id1-infected BMC in HGF, and have been maintained for over one year in culture. These cell lines have a phenotype that is representative of normal CMP and GMP progenitor cells. Furthermore, these cells are absolutely dependent on HGF for their growth and survival, and undergo further differentiation into granulocytes and macrophages in the presence of GM-CSF and G-CSF.